Inverter-less Thrust Magnetic Suspension Using Zero-Sequence Current in a 5-DOF Controlled Bearingless Motor

Five-degree-of-freedom controlled bearingless motor needs at least four inverters for levitation and rotation. For simplification of the system, this paper proposes inverter-less drive of thrust magnetic suspension. In the proposed method, the thrust suspension winding is connected between the neutral points of the three-phase motor windings and the three-phase radial suspension windings. The current flowing in the thrust winding is defined as zero-sequence current, resulting in generating thrust force without inverter for thrust suspension. The influence of zero-sequence current on the radial suspension force is investigated through theoretical equations and FEA, and can be suppressed by adopting small third-winding coefficient in radial suspension. The test machine was fabricated, and experimental results demonstrated that the thrust suspension was successfully performed by the zero-sequence current while four-axis motions are regulated in a non-contact manner during rotation.

I. Introduction

Bearingless motors generate torque while the rotor is suspended in a non-contact manner, therefore overcoming the challenges related to mechanical bearings. They have been applied to special machines such as turbomolecular pumps, flywheels, and compressors [1]. To achieve non-contact magnetic suspension, three translational (x, y, z) and two tilting (θ_x, θ_y) motions of the rotor must be magnetically stabilized by active or passive magnetic suspension.